See pic (from the Truth and Art TV twatter).
THE URL CANNOT BE CHANGED, it says. Let me deal with that point:
OK, maybe the URL to the image can't be changed, but the image/file AT that URL CAN be changed (remove old file and upload new file with the SAME FILENAME/URL) - using the Twatter Validation Tool forces the Twitter cache to clear, thus picking up the NEW file at that URL.
Just to add, you can have caches all over, Twitter's, your ISP may do some caching, your browser. Caching is where an item is stored locally (somewhere) so that it is retrieved quicker than going back to the originating server (in this instance) - there are several ways of clearing the cache (FORCING retrieval from the originating server, thus refreshing the locally stored item - if image has changed for example).
As for 'dynami'c - semantics imo. This is a way of semi-dynamically changing the image - in that it may require manual intervention to force the update).
This post is not to discredit Q (as the twatterer is saying), it's about TRUTH (regardless of who posted).